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Cassini Imaging Team/ISS/JPL/ESA/NASA

In a haze. The dense nitrogen atmosphere of Titan (foreground), which could have been generated by impacts of asteroids and comets onto an icy surface including ammonia, is a stark contrast to the airless moon Tethys (background), another of Saturn’s satellites.

Titan's Atmosphere Spawned by Impacts?

Titan's atmosphere may have resulted from an early pummeling. A new study bolsters the notion that the thick, nitrogen-rich atmosphere of Saturn's largest moon was literally blasted into existence billions of years ago by comets or other objects pounding its icy surface. The find may help solve a longstanding mystery about how and when that atmosphere came into being.

Titan isn't the largest moon in the solar system, but it is the only one with a dense cloak of gas. Atmospheric pressure on the moon's surface is about 50% higher than it is on Earth. More than 95% of the atmosphere is nitrogen, and where all of that nitrogen came from is one of astronomy's greatest mysteries, says Catherine Neish, a planetary scientist at Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland. "It's one of the big unknowns in the solar system," she notes. "You've got all these airless moons, and all of a sudden there's Titan."

Before the Cassini-Huygens mission arrived at Saturn in 2004, scientists weren't sure whether Titan and its atmosphere accreted simultaneously from the same cloud of material or whether the hazy, nitrogen-rich shroud appeared long after the moon was formed. Recent data gathered as the Huygens probe dropped through the atmosphere have pretty much ruled out the notion that those gases are primordial, Neish says in a commentary in Nature Geoscience that accompanies the new study. If the atmosphere had accreted with the moon, argon, which is heavier than nitrogen and thus even more likely to be trapped by a large body's gravity, should be a more prominent constituent of Titan's gaseous cloak. The dearth of argon strongly suggests that Titan lost its original atmosphere, if it had one in the beginning, Neish writes.

Some of the more-recent ideas about where Titan's nitrogen originated—from atmospheric ammonia that was broken apart by photochemical reactions, for example, or from the breakdown of ammonia in hydrothermal systems—require that the moon accreted at a relatively warm temperature. But that's another idea disproved by data gathered by instruments on board Cassini, Neish notes. One of the most likely hypotheses remaining, she suggests, is that the atmospheric nitrogen was generated when comets, asteroids, or other large objects slammed into Titan with so much energy that the ammonia-rich ices on its surface were chemically stripped apart.

To test this hypothesis in the lab, Yasuhito Sekine, a planetary scientist at the University of Tokyo in Kashiwa, Japan, and his colleagues fired tiny, laser-accelerated projectiles of gold, platinum, and copper at various speeds into mixtures of frozen ammonia and ice. The team's experiments suggest that the immense heat and pressure generated at impact speeds greater than 5.5 kilometers per second efficiently break down ammonia ice into nitrogen, hydrogen, and water vapor.

Between 3.8 billion and 4 billion years ago, most bodies in the solar system were incessantly pummeled by large numbers of asteroids, comets, and other large impactors—a beating that lends that period the moniker "Late Heavy Bombardment." Sekine and his team estimate that even if Titan were cold and airless at that time, the number and size of impacts expected for Titan would have been sufficient to generate the amount of nitrogen seen in the moon's atmosphere today. And if Titan had been relatively warm and swaddled in a primordial atmosphere, that gaseous cloak would have been blasted away and replaced with the ammonia-derived nitrogen, the researchers report online today in Nature Geoscience. In either case, today's dearth of atmospheric argon is explained.

"This is a great paper with an interesting idea," says Jonathan Lunine, a planetary scientist at the University of Arizona in Tucson. "What's really new here is being able to quantify the conversion of ammonia to nitrogen."

What's needed next, Lunine adds, are hypotheses and tests that would allow scientists to determine whether Titan's ammonia accreted during the moon's formation or was delivered much later—possibly during the Late Heavy Bombardment—by comets and other icy bodies.

Neish agrees, noting that a space probe sent to collect a sample of a comet and to measure its argon content would go a long way toward helping researchers determine the source of Titan's atmosphere.